[R] barplot(2?) with CI from a zero reference line

[Jean-Louis Abitbol]

Dear R Users, (and dear Marc)

First of all many thanks for the answers to my previous questions.

I would like to barplot the mean percent change of a variate with it's
CI. Bars should start from the zero reference line to height (in
barplot2).

Is there a way to tweak barplot2, for example,  to do that ? 

I have tried to see what the function was but unlike other functions was
not able to list it by > barplot2. Is it because it is called through
UseMethods ? 

Thanks for any help.

Jean-Louis